1. Technical Field
The present invention relates to a radiological image conversion panel, a manufacturing method of the radiological image conversion panel, and a radiological image detection apparatus.
2. Related Art
In recent years, a radiological image detection apparatus that utilizes a flat panel detector (FPD) detecting a radiological image and generating digital image data has been put into practical use. The radiological image detection apparatus has been distributed rapidly for the reason that an image can be instantly confirmed as compared with an imaging plate constituting with photostimulable phosphor (accumulative phosphor). Various types of radiological image detection apparatus are available and one of them is known as an indirect conversion type radiological image detection apparatus.
The indirect conversion type radiological image detection apparatus has a radiological image conversion panel and a sensor panel. The radiological image conversion panel has a scintillator which generates fluorescence when exposed to radiation and the sensor panel has a pixel array for detecting the fluorescence of the scintillator. The scintillator and the pixel array are bonded through an adhesive layer. The radiation transmitted through a subject is once converted into light by the scintillator, the fluorescence of the scintillator is converted into an electrical signal by the pixel array, and thus digital image data is generated therefrom. The scintillator and the pixel array, for example, are bonded by moving the roller and loading it on the supporting substrate of the radiological image conversion panel which supports the scintillator.
Scintillator typically includes alkali halide phosphors such as CsI (cesium iodide) or NaI (sodium iodide), and is composed of a group of columnar crystals in which crystals of the phosphors have been grown into columnar shapes on a support by a vapor deposition method. The columnar crystals formed by the vapor deposition method do not contain impurities such as a binder, and have a light guide effect that guides the fluorescence generated in the columnar crystals in a growth direction of the crystal so as to suppress the diffusion of the fluorescence. Thus, not only the sensitivity of the radiological image detection apparatus, but also the sharpness of the image can be improved (see, for example, Patent Document 1 (JP-A-2011-017683) and Patent Document 2 (JP-A-2003-066147)).
However, as in the scintillator formed out of the group of columnar crystals, because the fluorescence emitting surface to be bonded with the pixel array is composed by a set of the tip parts of columnar crystals and thus there are many empty places, it is difficult to get the bonding strength with the adhesive layer. Further, there might be occurred warpage at the radiological image detection apparatus, depending on the temperature variations of the environment and the temperature changes caused by the usage of the apparatus, due to the differences between the linear expansion rates of the supporting substrate that supports the scintillator of the radiological image conversion panel and of the sensor substrate that supports the pixel array of the sensor panel. In this case, there might be caused air gaps between the scintillator and the pixel array by their peeling. In addition, the air gap can be a factor of image defects or degradation of image sharpness by reflecting or scattering light. This peeling of the scintillator with the pixel array is prone to occur with starting from the edge portions of the scintillator.
Thus, in order to increase the bonding strength with the adhesive layer in the edge portions of the scintillator, a method in which the load applied on the edge portions becomes significantly higher compared to that applied at the center portion is considered when the scintillator is bonded with the pixel array. In addition, when the scintillator is bonded with the pixel array by loading with the rollers, the load applied on the edge portions of the scintillator usually becomes relatively greater. However, the crystals of the alkali halide phosphors such as CsI or NaI are hard and are also vulnerable, and specifically, each columnar crystal is prone to be broken, because there are pores around it and they are independent of the adjacent columnar crystal.
Thus, as in the radiological image detection apparatus described in Patent Document 2, the diameters of the columnar crystals in the edge portions of the scintillator become bigger than that in the center portion of the scintillator. Accordingly, because the strengths of the columnar crystals in the edge portions of the scintillator may become higher, the columnar crystals in the edge portions are prevented from being damaged by the load applied when the scintillator and the pixel array are bonded.
Further, as in the radiological image detection apparatus described in Patent Document 3 (JP-A-2010-025620), the fluorescence emitting surface of the scintillator to be bonded with the pixel array is flattened and at the same time the tip parts of the columnar crystals configuring the fluorescence emitting surface are integrated by filling between the tip parts of the group of columnar crystals with filler. Accordingly, because the strengths of the columnar crystals may become higher, the columnar crystals in the edge portions of the scintillator are prevented from being damaged by the load applied when the scintillator and the pixel array are bonded.
As in the manufacturing method of the scintillator in the radiological image detection apparatus described in Patent Document 2, when forming the scintillator by growing the crystals of phosphors by the vapor deposition method, the diameter distribution of the columnar crystals is done by controlling the internal temperature distribution of the surface of the supporting substrate on which the crystals are deposited. However, because the internal temperature distribution of the surface of the supporting substrate affects not only the diameters of the columnar crystals but also the lengths of the columnar crystals to produce a thickness distribution of the scintillator, it is very difficult to control the temperature distribution.
Further, as in the radiological image detection apparatus described in Patent Document 3, the filler is filled in an even depth between the tip parts of the group of columnar crystals throughout the entire scintillator. The light guide effect of the columnar crystal is to use total reflection due to the refractive index differences of the surrounding medium and the columnar crystal. If the filler is filled between the tip parts of the group of columnar crystals, the refractive index differences of the columnar crystals and the surrounding medium at the filled parts become smaller, and thus the light guide effect is weaken. Therefore, it is concerned that the image quality might be degraded.